Soil steam disinfection (SSD) technology is an effective means of eliminating soil borne diseases. Among the soil cultivation conditions of facility agriculture in the Yangtze River Delta region of China, the clay soil particles (SPs) are fine, the soil pores are small, and the texture is relatively viscous. When injection disinfection technology is applied in the clay soil, the diffusion of steam is hindered and the heating efficiency is substantially affected. To increase the heating efficiency of SSD, we first discretized the continuum model of Philip and De Vries into circular particle porous media of different sizes and random distribution. Then with Computational Fluid Dynamics (CFD) numerical simulation technology, a single-injection steam disinfection model for different SP size conditions was constructed. Furthermore, the diffusion pattern of the macro-porous vapor flow and matrix flow and the corresponding temperature field were simulated and analyzed. Finally, a single-pipe injection steam disinfection verification test was performed for different SP sizes. The test results show that for the clay soil in the Yangtze River Delta region of China, the test temperature filed results are consistent with the simulation results when the heat flow reaches H = 20 cm in the vertical direction, the simulation and test result of the heat flow in the maximum horizontal diffusion distance are L = 13 cm and 12 cm, respectively. At the same disinfection time, the simulated soil temperature change trend is consistent with the test results, and the test temperature is lower than the simulated temperature. The difference between the theoretical temperature and the experimental temperature may be attributed to the heat loss in the experimental device. Further, it is necessary to optimize the CFD simulation process and add the disintegration and deformation processes of soil particle size with the change of water content. Furthermore, the soil pores increase as the SP size increases and that a large amount of steam vertically diffuses along the macropores and accumulates on the soil surface, causing ineffective heat loss. Moreover, soil temperature distribution changes from oval (horizontal short radius/vertical long radius = 0.65) to irregular shape. As the SP size decreases, the soil pore flow path becomes fine; the steam primarily diffuses uniformly around the soil in the form of a matrix flow; the diffusion distance in the horizontal direction gradually increases; and the temperature distribution gradually becomes even, which is consistent with the soil temperature field simulation results. Similar to the energy consumption analysis, the maximum energy consumption for SP sizes>27mm and <2mm was 486and 477kJ, respectively. Therefore, proper pore growth was conducive to the diffusion of steam, but excessive pores cause steam to overflow, which increased energy consumption of the system. Considering that the test was carried out in an ideal soil environment, the rotary tiller must be increased for fine rotary tillage in an actual disinfection operation. Although large particles may appear during the rotary tillage process, an appropriate number of large particles contributes to the formation of a large pore flow, under the common effect of matrix flow, it will simultaneously promote greater steam diffusion and heating efficiency. The above theoretical research has practical guiding significance for improving the design and disinfection effect of soil steam sterilizers in the future.